Solomon Islands

Urban flood
Hazard level:
Medium

In the area you have selected (Solomon Islands) urban flood hazard is classified as medium based on modeled flood information currently available to this tool. This means that there is a chance of more than 20% that potentially damaging and life-threatening urban floods occur in the coming 10 years. Project planning decisions, project design, and construction methods must take into account the level of urban flood hazard. The following is a list of recommendations that could be followed in different phases of the project to help reduce the risk to your project. Please note that these recommendations are generic and not project-specific.

Climate change impacts: Model projections are inconsistent in their estimates of changes in rainfall. The present hazard level may increase in the future due to the effects of climate change. It would be prudent to design projects in this area to be robust to river flood hazard in the long-term.

Recommendations

EXISTING INFORMATION:
Obtain pre-existing flood hazard information. The high-level information available in this tool may indicate the presence of flood hazard in your project area. However, because flood hazard can change dramatically over short distances, the exact geographical location of your project should be checked against pre-existing flood hazard information.
More information

Acquisition of available flood hazard data should be undertaken for all sites and all hazard levels. Information about flood hazard can take multiple forms, including spatial maps of flood hazard, flood zoning information, reporting of previous events and local knowledge.

Local flood zoning information, typically from government planning departments, can indicate the likely flood hazard for specific locations. Local scale flood hazard maps can estimate the likelihood of flooding at a given location. Flood hazard maps may be available from government agencies or through local consultants and may be available at a variety of scales and with variable data quality. Additionally, urban planning authorities may indicate that some zones have critical drainage problems. Overwhelmed sewer and drainage systems can significantly contribute to the flood hazard in an area, so information regarding drainage issues should also be sought where available.

Most government-based flood hazard information will pertain to river floods, but some countries also produce surface water maps (flood hazard from intense rainfall). High-density developed areas are more prone to flooding from direct rainfall onto the land surface, since concrete surfaces force water to flow over the ground rather than infiltrate into it. Surface water flood maps should be acquired for such areas wherever possible.

In addition to local/country level flood hazard maps, regional and global flood hazard maps have emerged in recent years (e.g. SSBN, Aqueduct). These indicate the spatial extent of flood hazard over large scales but have important limitations. Flood models used to generate hazard data are subject to significant uncertainty and their output should be used tentatively. Flood hazard maps produced over large scales highlight where there is predominant flood hazard. However, owing to their coarse resolution, national or regional food hazard maps should not be used to provide information at local (building) scales and certainly not to inform detailed planning and engineering design. For this, localized flood modeling is required, with better topography and river channel data.

Observations of previous flood events can define flood hazard. Monitoring of flood events is becoming more routine, and archives of previous events now exist (Dartmouth Flood Observatory [http://floodobservatory.colorado.edu/], Disaster Charter [https://www.disasterscharter.org/web/guest/home], and Copernicus [http://www.copernicus.eu/]). The identification of previous events at a given location will clearly indicate the presence of a significant hazard. This can be used to refine the hazard levels provided by the ThinkHazard! tool. However, a location may have no events recorded in these archives; this does not mean there is no hazard – it simply means that an event has not yet been recorded officially.

Documentation of previous events, through local news reports, can also provide a useful insight into flood risk. Organizations such as Reliefweb (reliefweb.int) and FloodList (floodlist.com) provide information about previous events and their impacts. The local government may hold written records or observations into previous flood events. Likewise, residents near the project location may have a good understanding of the local flood behavior, particularly if they have resided there for a significant period. These sources of local knowledge should be used to further inform the likely flood hazard level at a given location.

TECHNICAL EXPERTISE:
Consult with professionals who can provide a more detailed understanding of the risk posed to your project by flooding. The level of guidance required will depend upon the level of hazard present, the vulnerability of the project and local legislation that might apply.
More information

o For medium to high vulnerability assets, consider commissioning a site-specific FRA, with the aim being to provide a detailed understanding of local flood risk.
o For lower vulnerability assets, consider commissioning a specialist flood risk appraisal, with the objective being to provide an overview of flood hazard in the region.

Professional guidance can range from informal advice to a full site-specific Flood Risk Assessment (FRA). The required level of consultation will depend on the vulnerability of the project or development, the anticipated flood hazard level and the level of FRA required (if any) by local legislation.
Informal advice can provide a greater understanding of flood hazard. Professional forums and even professional social-media platforms can provide access to professionals with expertise in flood risk assessment. Academics, with expertise in your location of interest, may also prove a useful resource. They may have intricate knowledge of flood hazard and could identify key datasets in the region.

A more detailed understanding of flood risk can be obtained via a flood risk appraisal. A flood risk appraisal provides a more localized view of flood risk (than provided by ThinkHazard!), but still a broad view of flood risk, highlighting key areas where a site-specific FRA may be required. These are typically desk studies undertaken by expert consultants that provide a generic assessment of flood risk by consolidating available information and perhaps undertaking coarse-scale modeling. Preferably, the consultant will have local expertise and be familiar with available data and relevant local legislation.

A site-specific FRA represents the most detailed appraisal of flood risk at a given location. FRAs include engineering-level site assessments, perhaps including detailed flood modeling. FRAs provide detailed flood risk information and inform the design process (the appropriate level of flood defense or site adaptation required). FRAs can be expensive and should be undertaken by expert consultants with extensive experience undertaking FRAs under the given climatology, topography, and be familiar with local legislation. If the project involves construction, building standards may apply to the project regarding flood risk, e.g. drainage for highways and embankment stability. Consultants will need to have appropriate skills in hydrology, hydraulics, and computational fluid dynamics. Always ask for detailed examples of relevant experience from any consultant you wish to hire for these purposes.

The level of professional guidance required will depend largely on the vulnerability of the project or development and the identified hazard level. High vulnerability assets are those that would be acutely impacted should they become inundated. For example, the Planning and Policy Statement 25 (PPS 25) in the UK, provides a classification of flood risk vulnerabilities that may be useful in determining the vulnerability of a project. PPS 25 defines partitions vulnerability into 5 categories:
Essential Infrastructure - all infrastructure that cannot be relocated, regardless of hazard level.
Highly vulnerable - assets that would acutely exacerbate the impacts of a flood, should they become inundated. Examples include emergency response units, hospitals, power stations, installations of hazardous materials.

More vulnerable - assets that would result in significant damage should they become inundated. Examples include residential buildings, educational and landfill facilities.
Less vulnerable - assets that would not significantly exacerbate the impacts of a flood, but would result in damages. Examples include shops, cafes, non-residential establishments, waste treatment (non-hazardous), agricultural buildings and land.
Water compatible - infrastructure that does not result in damages, during a flood. Examples include flood defenses, amenity open space, sports and recreational ground, docks and marinas.
A Highly Vulnerable asset, such as a hospital, will require a detailed understanding of flood risk. Therefore, for all hazard intensities greater than Very Low, a site-specific FRA would be highly recommended.

PROJECT LOCATION:
Consider relocation of the project. If local flood risk information confirms your project is exposed to a high level of flood hazard, relocation to areas not prone to flooding is recommended where viable. This decision will need to be undertaken with the consideration of other hazards.
More information

o Confirm hazard level using local data before considering relocation.
o Only water compatible, essential infrastructure and less vulnerable assets should be considered in areas with medium flood hazard. More vulnerable assets should be considered for relocation; this may be to a site nearby that has a lower flood hazard level, determined by local flood risk assessment (also consider other hazards reported in ThinkHazard!).
o Undertake cost/benefit analysis of relocation versus flood resilient design. Consider relocation if it is cost-effective and there are alternative sites available that still enable the project to meet its goals.

Depending on local flood hazard information and vulnerability of the project to flood, relocation of the project may become the most suitable and cost effective course of action to take, if the project goals can still be achieved from another location. Since this tool does not include information regarding local-level factors, upon which urban flood hazard is highly dependent, obtain a more detailed understanding of flood risk at the location of interest before considering relocation. Information concerning the drainage of surface water or details of structural defenses against river flooding are critical in determining flood risk at the site. If these indicate that the present area is at risk, it may be possible to relocate your project to a nearby site that is less susceptible to flooding. If local data confirm the presence of a flood hazard, undertake a cost/benefit analysis to weigh up whether relocation is preferable to a flood-resilient design.

When considering the relocation of a project, it is important to consider both the likelihood and the consequence of flooding. The likelihood of flooding is alluded to by the hazard levels provided in this tool. The consequence of flooding – the damage or loss of life that results from a flood – depends on factors including the vulnerability of the exposed asset. Understanding vulnerability, along with hazard level, is a crucial component when determining whether a location is suitable for development. High vulnerability assets are assets that would be acutely impacted should they become inundated. For example, the Planning and Policy Statement 25 (PPS 25) in the UK, provides a classification of flood risk vulnerabilities that may be useful in determining the vulnerability of a project. These classifications provide a basis to determine the vulnerability of the project or asset. PPS 25 partitions vulnerability into five categories:

Essential Infrastructure - all infrastructure that cannot be relocated, regardless of hazard level. Examples include water treatment works and essential transport infrastructure (including evacuation routes).
Highly vulnerable - assets that would acutely exacerbate the impacts of a flood, should they become inundated. Examples include emergency response units, hospitals, power stations, installations of hazardous materials.
More vulnerable - assets that would result in significant damage should they become inundated. Examples include residential buildings, educational and landfill facilities.
Less vulnerable - assets that would not significantly exacerbate the impacts of a flood, but would result in damages. Examples include shops, cafes, non-residential establishments, waste treatment (non-hazardous), agricultural buildings and land.
Water compatible - infrastructure that does not result in damages, during a flood. Examples include flood defenses, amenity open space, sports and recreational ground, docks and marinas.

EARLY WARNING SYSTEMS:
Identify early warning systems (EWS) that may exist in your project area. EWS aim to provide communities with advanced warning of an imminent flood event, based on weather forecasts, recorded rainfall or rising water levels upstream. They can be used to trigger protocols (such as the deployment of portable flood defenses or evacuation) to mitigate impacts of a flood event.
More information

Early Warning Systems (EWS) are a key tool that can reduce flood impacts. Upstream river levels and rainfall forecasts are commonly used to anticipate potential river and flash flooding. If sufficient warning can be given, then mitigation procedures can be implemented. These may include evacuation, movement of vulnerable assets/material or the implementation of temporary/moveable flood defenses. The availability of EWS may therefore significantly reduce flood risk.

EWS may vary from local scale procedures that already exist for your location, through to large scale EWS. Local EWS are likely to be run by government agencies, emergency responders or even local communities. Large-scale EWS also exist: for instance, GLOFAS is a global EWS for large scale river flood events. If the river system that poses a hazard is represented, then GLOFAS can be used to provide a provisional EWS. Large-scale EWS for flash-flooding is not widely available, though there are systems currently in the early stages of development (e.g. FloodCitiSense).

o Identify whether an EWS exists for the project area. This is likely to be run by the government agency responsible for rivers but other stakeholders, such as emergency responders, reservoir or hydroelectric plant operators, or even community run systems may exist.
o Global EWS may be applicable, check whether the hazardous river systems are included in GLOFAS (http://www.globalfloods.eu/). GLOFAS will likely only be applicable to large-scale river systems.
o Ascertain the feasibility of a real-time connection to the EWS. This can be as simple as receiving a text message or email from the EWS operator when a flood is expected.
o Ensure that any received early warning can be rapidly and clearly disseminated to all staff at the project location
o Develop protocols that define the actions to be taken when an early warning is received. Depending upon the sophistication of the EWS, a range of protocols may be required to define actions for different warning levels. Ensure that staff are aware of the EWS, understand how long they will have to respond to a warning, and are familiar with the response protocols.
o For critical or networked assets, protocols should warn dependents of possible service interruption.
o For critical or networked assets, protocols should warn backup assets that their service(s) may be required imminently.

LOCATION ASSESSMENT:
The high-level information available in this tool may indicate the presence of urban flood hazard in your project area. Before committing significant resources to this issue, a study of the immediate environment can help you to assess whether more detailed assessment and/or intervention should be considered.
More information

Flooding in urban areas is a very local phenomenon, so the hazard information provided by this tool should be considered the preliminary action in defining a flood hazard level. Flood hazard can vary dramatically over short distances, depending on local topography, drainage capacity and distance to waterways.

A local assessment should be undertaken to identify any obvious sources of flooding (rivers/streams). A lack of rivers or streams nearby does not rule out the possibility of flooding resulting from direct rainfall onto the land surface. Identify whether any structural flood defenses are in place: particularly levees (large structures preventing rivers overtopping their banks) and storm drains (systems designed to remove large volumes of water from the land surface in an area of interest). Location assessment can provide a greater understanding of the likely flood hazard, through the interpretation of both natural and human aspects of the local environment.

o Be aware that the hazard level provided by this tool is a first estimate of urban flood hazard. Further clarification of the local flood hazard will be required.
o Assess the local topography (i.e. how flat or steep your area is). Flooding is most likely to occur in relatively flat areas beside rivers (‘floodplains’), in local low-points in the land surface (‘depressions’), or along valley bottoms. If your project is situated at the base of a hill or on flat terrain it is more likely to be at risk than if it is situated on the top or side of a hill.
o Check for streams and rivers near to your project area. You can use tools such as published maps, Google Earth, or open source GIS software such as QGIS, to identify waterways. Do not dismiss small streams – these can swell rapidly and dramatically in flood conditions and cause serious local flooding. In some areas, rivers may not run all year around; such ‘ephemeral’ rivers are still capable of causing significant flooding at certain times (this type of river is most common in arid and semi-arid regions).
o Flash-flooding may arise from proximity to small streams and/or from intense rainfall directly onto the land surface. If excess rainfall cannot infiltrate the land surface it will flow over it, generating flood hazard. This phenomenon is often prevalent in high-density developed areas, where impervious surfaces (e.g. concrete) force water to flow overland.
o If your location of interest is in a high-density developed area (e.g. in a city), assess the existence and quality of local flood defense structures. The presence of a levee, how often it is maintained, and the size of the flood it defends against, will indicate the level of flood risk at a location of interest. Storm drains should be similarly investigated, especially ensuring there are no blockages in the drains. This information may be provided by local or national governments.
o Identify debris from previous floods. The presence of large objects, such as boulders, near to even small waterways may indicate the presence of powerful flood waves. Such behavior is particularly prevalent in small, steep tropical catchments.

o Attempt to identify the origin of the hazard identified by the tool. This could be a nearby large river system, or may be due to flooding resulting from intense local rainfall directly onto the land surface.
o Note that river flood risk may not always originate from the closest point on a river; flood waters can overflow from the channel upstream before flowing downhill over the floodplain.
o Identify whether there are any additional sources of flood hazard present, such as ‘ephemeral’ rivers and coastal flooding, which is reported as a separate hazard in this tool – if the location of interest is close to sea level in a coastal zone then an investigation into coastal flood hazard is recommended.

FLOOD HAZARD MANAGEMENT: Your project or development should consider flood management measures, such as catchment management and implementation of flood defenses.
More information

o Identify whether flood management procedures are in place in the local region, and how your development will integrate within the existing flood management framework.
o For large developments, consider implementing flood management procedures, such as the provision of storage areas for excess river flow, or the implementation of measures to reduce rainfall run-off.
o Consider implementing an Integrated Flood Management approach where possible.
o If your project provides a critical service, consider implementing basic measures to ensure the project can continue to function in the event of a flood, such as locating critical electronic equipment above ground level.

o Undertake a cost-benefit analysis to assess including moderate flood defense features.
o Consider ensuring accessibility to the project in the event of flooding.
o Consider flood-resilient design if the asset provides a critical service. Flood resilient design enables a building to continue to function in the event of a flood. Typically, it involves raising all critical services to above the maximum plausible flood height.
o Consider ensuring your site possesses a drainage system capable of transporting excess floodwaters away from your site, being mindful of how this may impact flood hazard elsewhere.
o Consider using cost-effective sustainable urban drainage systems (SUDS) principles such as permeable concrete

Flood management includes all measures that may alleviate flood risk. This can range from management of upstream catchment areas through to the implementation of flood defenses. Flood management is likely more applicable to projects in low-density developed areas (e.g. rural areas or the outskirts of towns and cities), as flood management in more built-up urban areas is often under the control of local and national authorities. Management of upstream catchment areas is sometimes defined as being ‘Green’ protective measures, as they often involve the restoration of natural wetlands or the removal of impermeable surfaces. Some examples of Green protective measures include:
-Measures to reduce runoff: replacing impermeable surfaces (e.g. concrete) with natural landscapes and afforestation are measures that will reduce the rate at which rainfall moves across the surface of a catchment and into a river.
-Storage of runoff: wetlands and reservoirs can store excess water during extreme flows, reducing the magnitude of the flood event. Often the restoration of natural wetland areas will alleviate flood risk, whilst also providing additional benefits to local eco-systems.
The implementation of flood defenses is sometimes referred to as ‘Grey’ protective measures, as they involve the implementation of built structures to protect areas or transport water away as quickly as possible. Examples of Grey measures include:
- Capacity enhancement of rivers: bypass channels and channel deepening/widening are measures that can be used to increase the amount of water that can pass through a river channel.
- River flood defenses: levees can be built to separate the hazard source (river) from the receptor (e.g. a building).
- Flash-flood defenses: storm drains can be constructed to enable the rapid removal of excess surface water from an area of interest. Permeable concrete could be used as part of a sustainable urban drainage system (SUDS).

The flood management options available will depend upon the size of the development, the resources available and the willingness/ability of other land users in the catchment area to participate in a flood management scheme. In the case of large developments, encompassing significant proportions of a river catchment, a number of flood management procedures will be available, including measures to reduce and store floodwaters. For small-scale developments, the implementation of upstream, flood alleviation measures may not be possible. Instead, the implementation of flood defenses, aiming to protect the structure or development from a floodplain, or the development of flood resilient structures, may represent the available options. Sustainable Urban Drainage Systems (SUDS) are designed to reduce the impact of new urban developments on surface water drainage. These systems take a holistic view of storm water management and include measures to increase infiltration (such as permeable concrete), detain and store flood water (such as surface wildlife ponds or subsurface storage tanks) and increase evapotranspiration (‘green’ roofs). Further information on SUDS can be found at http://www.susdrain.org/. Regardless of the measures implemented, it is important when implementing flood management measures that they are not treated in isolation; their impact on other areas may be significant.

The World Meteorological Organization (WMO) identified that attempts to control flooding via traditional Grey methods in single locations or developments have had little success in reducing flood losses and damages. Instead, the WMO has outlined what it calls Integrated Flood Management. Integrated Flood Management moves away from measures of flood control, such as engineered flood defenses in single locations, and instead, promotes the management of the water cycle as a whole. This approach not only aims to alleviate flood risk in highly vulnerable areas but also aims to create opportunities from flooding by maximizing the net benefits from the use of floodplains. This may include restoring natural floodplain areas or replenishing agricultural land. Where possible, this integrated approach to flood management represents the ideal approach to alleviate flood risk. However, such approaches will likely require a framework for collaboration across many different sectors and organizations. The Associated Programme on Flood Management provides documentation, guidance and test cases outlining how to implement Integrated Flood Management strategies.

DO NOT INCREASE HAZARD:
Built infrastructure may alter flood hazard. Constructing a significant piece of infrastructure can significantly alter the landscape and potentially influence how an area responds during a flood. Any alteration of the landscape should be undertaken with consideration as to how this will influence the local hydrology.
More information

o Consider how your development may affect local drainage behavior.
o Consider how your development affects local drainage behavior and its implications for flood risk in downstream areas. For instance, increasing impermeable paved areas (preventing water passing through to the soil) through development can increase flood hazard by increasing the amount and speed at which water drains from your site.
o Check to see if other planned projects in your area or upstream may affect your project, for example by moving water downstream to your site more quickly thereby increasing flood hazard at your planned site.
o Consider getting guidance on how to integrate your development into local flood management practices (see the Associated Programme on Flood Management).
o Be aware of the potential impacts of your development on local ecosystems.

o Implement sustainable drainage techniques, ensuring limited impact on other areas (See Associated Programme on Flood Management and PPS 25).
o Some form of Flood Risk Assessment is recommended to gain a greater understanding of the local drainage behavior and inform sustainable drainage design.
o If applicable, consult other river users about your plans.

When building a new development, consider how it will impact local hydrology. Built infrastructure can significantly change how an area responds to rainfall and how water is transported, potentially increasing flood risk. For example, replacing large permeable areas with impermeable surfaces or structures will increase the amount of excess water that flows over the land surface rather than infiltrates into it. Sufficient drainage systems will need to be in place to transport the excess water that previously would have been absorbed by the landscape, away from the site. However, this will increase the speed at which rainfall enters local river networks, potentially magnifying flood risk in downstream areas.

This section is less applicable to high-density developed areas, where an intense built environment will have already altered flood hazard (i.e. it is likely your development will not exacerbate this effect). If your project will contribute to urbanization of low-density developed areas, its effect on flood hazard may be more significant.

Obstruction of waterways, for example with a poorly located bridge, can increase adjacent, as well as upstream, river flood risk. This will create the potential for flooding to occur more regularly and to a greater magnitude. Any development that encroaches upon the drainage capacity of a channel can also magnify flood risk by allowing waterborne debris to gather, further restricting flow.

Buildings and developments also have the potential to exacerbate downstream river flood risk by reducing floodplain storage capacity. If a development removes space that previously stored excess water, the scale and/or speed of run-off will increase. These considerations also apply to flood defenses and changes in channel capacity. The implementation of river flood defenses will, by its nature, remove some amount of floodplain storage and exacerbate flood risk elsewhere. Similarly, measures to increase river channel capacity, with a view to making an area more drainage efficient, will likely increase the magnitude of flooding downstream.

Developments can increase the likelihood of flash flooding in an area: impervious surfaces (e.g. concrete) force excess water to flow over land rather than infiltrate into it. Extreme rainfall is, therefore, more likely to translate into surface water flooding than if the land surface was kept permeable. The installation and maintenance of urban drainage systems (e.g., storm drains), can effectively remove water from the surface; however, by decreasing the time it takes for rain water to reach the river channel, such systems can lead to an increase in river flood risk downstream. Where drainage systems are poorly maintained, blockages can occur and lead to increased surface flooding. It is therefore essential that provision is made for the maintenance of any new drainage systems.

Aside from direct human impacts, the impacts of a development on local flood behavior should also be considered from an environmental viewpoint. Floodplains are valuable ecological resources that will provide a habitat for a wide variety of plants and animals. All developments should be considered in terms of their potentially negative impact on local ecosystems, either through the removal of natural floodplain areas or through the amplification of flooding.